Method and apparatus for welding with output stabilizer

ABSTRACT

A short circuit arc welding system is disclosed. The control scheme uses a current command signal to drive the output current. The command signal is comprised of a long-term current command that sets the long-term current command level and a real-time or short-by-short current command. Arc voltage feedback is used to determine if the desired arc length is present and to adjust the long-term command. The short-by-short current command is derived from real-time arc current feedback and is used to control the burn-off rate by an instantaneous, or short-by-short, adjustment of the current command. A function of the time derivative of arc power, less the time derivative of arc current, is used to detect, in real time, when the short is about to clear. A stop algorithm is employed that monitors the arc on a short-by-short basis. When the process is ending a very low current level is provided to avoid forming a ball. However, if a short is created, (indicated by a drop in arc voltage) after the low current level, a burst of energy is provided to clear of burn off the short. After the short is cleared, very low current is again provided to avoid forming a large ball. This is repeated until the wire stops and the process ends.

This is a continuation application Ser. No. 09/526,770, filed Mar. 16,2000, now U.S. Pat. No. 6,326,591 entitled Method Apparatus for ShortArc Welding, which is a continuation of application Ser. No. 09/024,944filed on Feb. 17, 1998, which issued as U.S. Pat. No. 6,087,626.

FIELD OF THE INVENTION

The present invention relates generally to the art of welding powersupplies. More specifically, it relates to welding power supplies andthe control thereof for short circuit welding.

BACKGROUND OF THE INVENTION

There are many types of welding power supplies and welding processes.One welding process is referred to as short circuit transfer welding.Short circuit transfer welding generally consists of alternating betweenan arc state and a short circuit, non-arc state. During the arc statethe wire melts, and during the short circuit state the metal furthermelts and the molten metal is transferred from the end of the wire tothe weld puddle. The metal transferred in one cycle is referred toherein as a drop, regardless of the size or shape of the portion ofmetal that is transferred.

Short circuit transfer welding has many advantages, such as shorter arclength and less melting of the base plate. However, short circuittransfer welding has disadvantages, such as increased spatter.

Both the power source topology and the control scheme must be consideredwhen designing a short circuit transfer welding power source. The powertopology used must be fast enough to have a timely response to thechosen control scheme. The control should address three considerations:First, arc length must be properly controlled. Second, the burn-off (ormass deposition) rate must be appropriately controlled. Inappropriateburn-off rate will result in increased spatter. Third, spatter is alsocaused by too much power when the short is cleared, i.e., the transitionfrom a short circuit to an arc. Thus, the power or current when theshort clears must also be controlled. Also, when the short is about toclear must be detected. Some prior art patents do not teach control ofthe short circuit transfer welding process on a short circuit by shortcircuit basis. Such a control will provide more precise control of thewelding process and will help to reduce spatter.

One common prior art power source topology uses secondary switchers tocontrol the output. While these may provide fast control, they may berelatively expensive or have insufficient peak current capacity. Also,switching high current may increase reliability problems and switchinglosses. Examples of patents that have secondary switchers include: U.S.Pat. No. 4,469,933, entitled Consumable Electrode Type Arc Welding PowerSource, issued Sep. 4, 1984; U.S. Pat. No. 4,485,293, entitled ShortCircuit Transfer Arc Welding Machine, issued Nov. 27, 1984; U.S. Pat.No. 4,544,826 entitled Method and Device For Controlling Welding PowerSupply to Avoid Spattering of the Weld Material, issued Oct. 1, 1985;U.S. Pat. No. 4,717,807, entitled Method and Device For Controlling aShort Circuiting Type Welding System, issued Jan. 5, 1988.

The control scheme in many prior art power supplies uses arc voltage todetermine if arc length is proper. Typically, if the arc voltage is lessthan a setpoint, the arc length is determined to be too short, and ifthe arc voltage is greater than the setpoint, arc length is determinedto be too long. The output current is controlled to either increase ordecrease the amount of metal being transferred, thus controlling the arclength. Some prior art short circuit transfer welding patents taughtcontrol of the mass deposition (burn-off) rate by controlling thewelding power by “totalizing” the energy delivered to the arc. Arc orwelding power is a function of arc current and arc voltage.

However, the burn-off rate on a short-by-short basis (i.e. for any givenshort circuit transfer welding cycle) is largely independent of arcvoltage—it is predominantly a function of arc current. Thus, prior artcontrol schemes that use arc power (or arc energy) to control theburn-off rate are complex, and inaccurate. Example of such complex andinaccurate control schemes include: U.S. Pat. No. 4,866,247, entitledApparatus and Method of Short Circuiting Arc Welding, issued on Sep. 12,1989; U.S. Pat. No. 4,897,523, entitled Apparatus and Method of ShortCircuiting Arc Welding, issued on Jan. 30, 1990; U.S. Pat. No.4,954,691, entitled Method and Device For Controlling A Short CircuitType Welding System, issued on Sep. 4, 1990; and U.S. Pat. No. 5,003,154entitled Apparatus and Method of Short Circuiting Arc Welding, issued onMar. 26, 1991. Some of these prior art patents teach control of thepower when a short is clearing by predicting the clearing of the short.They generally compare arc voltage or its first derivative to athreshold. However, the prior art attempts result in missed or falsepositive short clearing predictions.

Accordingly, a short circuit transfer welding power supply thatadequately controls the burn-off rate, preferably on a short-by-shortbasis, is desired. Preferably, the process should be controlled suchthat power is reduced when the short is clearing. Also, the power sourceused should be sufficiently fast to respond to the control, but notunduly expensive or limited in peak output current.

One of the causes of instability in a short circuit transfer weldingprocess relates to excessive pre-heating of the wire. Variations in thewire/puddle interaction caused by operator movement and/or changingpuddle geometry, can result in irregular pre-heating of the wire due toI²*R heat generation. Too much pre-heating of the wire can cause themelting rate of the wire to increase to a point where the molten ballgrows very quickly following the transition from a short to an arc. Thisquick melting, known as a flare-up, results in a rapid increase in arclength with a corresponding voltage increase.

The opposite extreme can also occur. If there is insufficientpre-heating of the wire, the short circuit frequency will increase assubsequent arc times become shorter. If energy is not added quicklyenough, the wire can eventually “stub” into the puddle. The end resultof such stubbing is either an explosive short clearing, or a sustainedshort circuit with no arc (sometimes called noodle welding). Over andunder preheating often occur in a cyclic fashion. Unfortunately, mostprior art controls adjust after a stub or flare-up has occurred. Forexample, when the control causes the heat to decrease to compensate forpast pre-heating, the process has already cycled to the under-heatingstage. Thus, the control actually exacerbates the problem. Accordingly,it is desirable to have a short circuit transfer welding process thataccurately compensates for the pre-heating of the wire.

It is desirable to have consistent arc starting in most weldingprocesses. The size of the ball at the end of the wire (formed when thelast weld was terminated) is a significant factor in determining theamount of energy needed to initiate the arc. Thus, the condition of theend of the wire (size of the ball) from the previous weld should beconsistent to provide consistent arc starting.

However, the size of the ball can vary from 1 to 3 times the diameter ofthe wire after a typical short circuit transfer welding process hasended. Previously, sometimes an operator cut the end of the wire, whicheliminated the ball, or in some prior robotic arc spray systems an extrastep to dress or trim the wire at the end of each weld and to insure thewire isn't frozen to the welded joint at arc end is provided (U.S. Pat.No. 5,412,175 issued May 2, 1995, e.g.). While this may produce auniform wire diameter at the start of the next weld, it wastes time, andthe extra step would not be needed if the wire had a consistent diameterwhen each weld is stopped.

There have been attempts in the prior art to control the termination ofa welding process. A BETA-MIG® has used a predetermined “crater” for thestops. However, the BETA-MIG® did not provide a fast enough response, oran adequate control scheme, to produce the consistent ball size desiredfor short circuit transfer welding.

Another prior art system is in the Miller 60M® pulsed spray process,which has an algorithm that reduces the output pulse frequency to matchthe stopping of the motor. A final pulse is sent which blows one lastball off the wire and extinguishes the arc. However, this method willnot work for processes such as short circuit transfer welding, that donot tightly control the frequency of the output power. Also this priorart does not desirably compensate for irregularities in the process,such as unintended shorts.

Accordingly, a power source and controller that provide a stop algorithmthat reduces the size of the ball to be about that of the wire diameter,or of a size that allows consistent starts to be made, i.e. not a largeball, when the process is terminated, is desirable. This process will,preferably, insure that the wire is not frozen to the weld joint at arcend. Also, the stop algorithm should preferably be robust (i.e. able tofunction even during irregularities in the process) and adaptable for avariety of processes, such as MIG processes, spray processes, pulsedspray processes, or short circuit transfer processes.

SUMMARY OF THE PRESENT INVENTION

According to a first aspect of the invention, a welding process andapparatus includes depositing drops of molten metal at the end of awelding wire into a weld puddle. A power source has a current output inelectrical communication with the welding wire. A feedback circuitprovides a real-time signal indicative of the heat input to each drop. Acontroller is coupled to the power source and has a feedback inputcoupled to the feedback circuit. It controls the magnitude of thecurrent provided to the welding wire in response to the heat of eachdrop.

One aspect of the invention is that the feedback includes a currentsignal representative of the output, and the controller determines thepower delivered to the wire. The controller also determines when theshort is about to clear in response to the power delivered. Thecontroller may determine a rate of change of the output power.

Another aspect is that the controller determines a value V_(c) definedby V_(c)=k*(dP/dt), where V_(c) is a calculated value, k is a scalar,and dP/dt is the derivative of the power. The controller compares V_(c)to a threshold. The controller subtracts a value responsive to the rateof change of the output current from the rate of change of the outputpower, in another embodiment.

The controller takes the derivative of a value responsive to the rate ofchange of the output power less the value responsive to the rate ofchange of the output current, in another embodiment. Also, thecontroller determines a value V_(c) defined byV_(c)=d/dt(k1*dP/dt−k2*di/dt), wherein k1 is a scalar, dP/dt is thederivative of the output power, k2 is a scalar, and di/dt is thederivative of the output current.

The controller provides a desired mass deposition rate responsive to awire feed speed and a distance from a tip of the wire to the workpiece,in another alternative.

The controller compares a value responsive to the energy needed todeposit a given amount of wire to a value representing the amount ofenergy delivered in at least a portion of one welding cycle, in anotherembodiment. The controller determines the energy needed in accordancewith Q_(req)=k3 (R_(dep) * (H_(m)+(T_(drop)−T_(amb)) * C_(p)) *t_(tot)), where Q_(req) is the energy needed, k3 is a scalar, R_(dep) isa wire mass deposition rate, H_(m) is a latent heat of melting for thewire, T_(drop) is the temperature of the molten drop, T_(amb) is theambient temperature of the wire, C_(p) is the heat capacity of the wire,and t_(tot) is a cycle length. The controller determines the energydelivered in accordance with Q_(wire)=((V_(anode)+WF+3 kT/2 e) *I+I²*l*rho/A), where Q_(wire) is the energy delivered, V_(anode) is theanode voltage drop, WF is the work function of the metal comprising thewire, (3 kT/2 e) is the thermal energy of electrons impinging on thewire, I is the output current, l is the contact tip to arc distance, rhois the resistivity of the wire, and A is the cross sectional area of thewire.

The controller determines a length of stick out (i.e., the length of thewire that extends from the contact tip), in another embodiment. Stickout is determined by providing an arc voltage setpoint, and comparingthe arc voltage setpoint to the arc voltage. Then the comparison isintegrated over time. The integrand is summed with an integrated burnrate error, and the sum is compared to known values.

Another embodiment includes stopping the welding process. The status ofthe arc is monitored, and the current is increased in response to theforming of a short circuit. Then, the current is driven to a low currentlevel when the short has cleared, such that a large ball at the endof-the wire is not formed. This is repeated until a short does not occurand the wire stops.

Another embodiment provides that the wire feed speed is monitored, andthe stopping of the process begins when the wire feed speed drops belowa threshold. In various embodiments the welding process is a MIG, spray,pulse spray, globular or short circuit transfer welding process. Inother embodiments the arc is monitored by monitoring the arc voltage.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing current and voltage outputs for a shortcircuit transfer welding cycle;

FIGS. 2A and 2B are circuit diagrams showing part of a controller thatdetermines when the short is about to clear;

FIG. 3 is a graph showing current and voltage outputs, and a feedbacksignal created by the circuits of FIGS. 2A and 2B;

FIGS. 4A & 4B are circuit diagrams showing part of a controller thatsets the current command;

FIG. 5 is a cross sectional diagram of a contact tube and welding wire;

FIG. 6 is a graph showing wire feed speed and oscillation frequency fora MIG short circuit transfer welding system;

FIG. 7 is sectional diagram of the stick out portion of a welding wireused in a MIG short circuit transfer welding system;

FIG. 8 is a block diagram of a MIG short circuit transfer weldingsystem; and

FIG. 9 is a circuit diagram of an active stabilizer used in thepreferred embodiment.

Before explaining at least one embodiment of the invention in detail itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting. Like referencenumerals are used to indicate like components.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention will be illustrated with reference to apreferred control scheme, a preferred control circuit, a preferred powersource and illustrative waveforms, it should be understood at the outsetthat the invention is not limited to the components described herein.Other circuitry and control schemes may be employed while implementingthis invention.

A method and apparatus for controlling a short circuit (MIG) weldingprocess is described herein. A wire electrode is mechanically fed intothe weldment at a relatively constant rate by a wire feeder in the shortcircuit transfer welding process. It is consumed into the weldment via aseries of alternating short circuit and arc events. This process isgenerally referred to as short circuit welding, or short circuittransfer welding. Generally, a welding machine used for short circuitwelding includes at least a power source, a controller and a wirefeeder.

The short circuit transfer welding process is cyclical. One cycle of theprocess, as described herein, begins with the beginning of a steadystate arc, followed by a short circuit condition, and is completed withthe beginning of another steady state arc condition. A typical cyclelength is 10 msec. The electrode, and a portion of the base metal, aremelted during the short circuit transfer welding process by currentflowing through the electrode to the weldment. Generally, a portion ofthe wire material melts during the arc condition, and is transferredduring the short condition.

FIG. 8 is a block diagram of a MIG short circuit transfer welding systemthat implements the present invention. Generally, a wire feeder 801provides a wire 802 through a welding torch 804 to a weldment 803. Apower source 805 provides power to welding torch 804 and a workpiece806. A controller 807 includes a microprocessor 808 (an 80196KC)microprocessor in the preferred embodiment, and a DSP or otherintegrated circuit in alternative embodiments), an A/D and D/Ainterface, and an analog circuit 809. Feedback is provided to controller807 on lines 811-813. Control signals are provided by controller 807 onlines 814-816. Controller 807 may be part of power source 805, part ofwire feeder 801, power source 805 may have a separate controller, orcontroller 807 may directly control the power converting of power source805.

The preferred control scheme uses a current command signal to drive theoutput current. The command signal is comprised of multiple components.One component sets the long-term current command level (called thelong-term current command). Another component adjusts the currentcommand on a real-time or short-by-short basis (called theshort-by-short current command).

Arc voltage feedback is used to determine if the desired arc length ispresent and to adjust the long-term command. The short-by-short currentcommand is derived from real-time arc current feedback (rather thanpower) and is used to control the burn-off rate by an instantaneous, orshort-by-short, adjustment of the current command.

The preferred control scheme also uses a function of the time derivativeof arc power (less the time derivative of arc current) to detect, inreal time, when the short is about to clear.

A stop algorithm is employed that monitors the arc on a short-by-shortbasis. When the process is ending a very low current level is providedto avoid forming a ball. However, if a short is created, (indicated by adrop in arc voltage) after the low current level, a burst of energy isprovided to clear the short. After the short is cleared, very lowcurrent is again provided to avoid forming a large ball. This isrepeated until the wire stops and the process ends.

The preferred embodiment uses a power source which has the capability tochange its' output current very rapidly, on the order of 1000 amps/msec.One example of this type of power source would be an inverter powersource system with a low output impedance, or a secondary switcher.

The specific power source of the preferred embodiment of this inventionis a series resonant convertor, such as that described in U.S. patentapplication Ser. No. 08/584,412, filed Jan. 11, 1996, entitledSwitchable Power Supply With Electronically Controlled Output Curve AndAdaptive Hot Start, which is hereby incorporated by reference. Thepresent invention uses a controller (described below) that controls thewelding process and cooperates with the power source. The controllerdescribed below provides a command to the power source indicating thedesired current magnitude. The power source includes its own controllerwhich causes the power source to provide the desired current. The powersource is controlled by an external controller (that also implements thecontrols described herein), in another embodiment. The series resonantconvertor is preferred (but not required) because it has a very fastresponse to the desired current command. Other embodiments use othertypes of power sources, including invertors, phase controlled, andsecondary switcher power sources.

The invention described herein includes algorithms implemented bymicroprocessor 808 and analog circuit 809. Implementing the algorithmentirely with discrete components, or entirely with a microprocessor,DSP, or other integrated circuits are alternative embodiments. Thealgorithms control the welding process by controlling the wire burn-offor mass deposition rate on a short circuit by short circuit basis. Thewire burn-off rate is controlled by controlling the current on a shortcircuit-by-short circuit basis (or period-by-period basis). Thisshort-by-short current control is combined with the current control setby arc voltage (to obtain a-desired arc length). The power source andcontroller of the preferred embodiment are sufficiently fast to providethe desired current in much less than one weld cycle.

The short-by-short burn-off rate is controlled, in the preferredembodiment, based on arc current feedback. Arc voltage is not used, inthe preferred embodiment, to control the short-by-short burn-off ratebecause wire burn off-rate is dependent on arc current rather than arcvoltage.

Thus, two control loops are in simultaneous use—an arc length loop usingarc voltage as feedback to set a long-term current command, and a wireburn-off loop using arc current as feedback to set a short-by-shortcommand. The two loops are weighted differently in the preferredembodiment. Both arc voltage and arc current are used to detect, in realtime, the short-clearing, and to terminate the process, as describedbelow.

It is easiest to understand the circuitry and algorithm used toimplement the preferred embodiment by referring first to typical outputvoltage and current waveforms, such as those depicted in FIG. 1. Thedashed lines indicate time segments which are referred to as T_(back) inT_(wet), T_(rise1), T_(rise2), T_(dpdt), and T_(hld). These timesegments indicate when, in the current waveform, changes are effected bythe algorithm.

T_(hld) is an arc condition that begins at the end of the shortclearing. The current is commanded to a level high enough to melt theend of the wire during T_(hld). T_(hld) is maintained for a durationlong enough that a desired amount of heat (or energy) is input into thewire. When T_(hld) ends, T_(back) begins.

T_(back) is a steady state arc condition. During T_(back) the current isat a background level, A_(bk), which is sufficient to sustain an arc.However, the background current is not of a sufficient magnitude tocontinue to melt the wire faster than the rate at which it is being fedinto the weldment. The arc condition with a low background current endswhen the tip of the wire makes contact with the weld puddle, which isdenoted by the end of T_(back) and the beginning of the T_(wet) time. Ifthe short does not occur during T_(back) after a certain length of time,the current is lowered to an even lower background level to make surethat eventually a short will occur.

The end of the arc condition is also the beginning of the short circuitcondition. This transition causes an abrupt drop in the output voltage.The algorithm of the preferred embodiment sets the beginning of theshort as the time at which the output voltage crosses a threshold,V_(sht). The threshold is set by a comparator that receives a voltagefeedback signal and provides its output to controller 807. The thresholdmay vary depending upon wire feed speed, wire size and type, and/orother weld parameters.

The inventors have discovered that a high current level at the inceptionof a short circuit may cause a “whisker short”. A “whisker short” is ashort circuit of abnormally short duration because the initial contactpoint between the wire and puddle is not sufficient to handle thecurrent magnitude (i.e. it acts almost like a fuse that blows). Thus,the current is normally decreased at the start of each short circuit, inthe preferred embodiment. Alternative embodiments do not use thetemporary decrease in current.

The decrease is accomplished by microprocessor 808 changing the currentcommand by a factor of Dip % at the onset of the short circuit (i.e. thebeginning of T_(wet)). Thus, the current command during T_(wet) (calledA_(wet)) is defined as A_(wet)=A_(bk)*Dip %. Dip % is typically lessthan one (and as low as zero) to insure that the molten metal on the endof the wire wets into the puddle. However, Dip % can also be greaterthan one, and can be dependent on the wire feed speed. The lowered(A_(wet)) current level is maintained for the period called T_(WET) toinsure the molten material on the end of the wire transfers into thepuddle.

The duration of T_(wet) is dependent upon the size of the molten balland is therefore dependent on wire feed speed. Also, changes in thecontact tip to work distance induced by the operator in a semi-automaticoperating mode can cause variations in the size of the molten ball fromone short circuit sequence to the next. Thus, the duration of T_(wet) isresponsive to wire feed speed and operator movement.

The inventors have learned that, generally, the size of the molten ballmay be correlated to the duration of the prior arc time. The longer agiven arc lasts, the greater the amount of wire melted which must betransferred during the next short circuit. Microprocessor 808 monitorsthe time of each arc and compares that time to a preset nominal arctime. The difference between the two values is used to effect a changein the length of T_(WET). More specifically, according to the preferredembodiment, if a given arc sequence exceeds the preset nominal value,then T_(WET) is increased by an amount proportional to the differencebetween them. The algorithm defines T_(WETNew) asT_(WETOld)+WETtgain*(T_(arcset)−T_(arcactual)).

When T_(WET) is completed the current is commanded to increase. Thisportion of the current waveform is called T_(RISE1). The rate of rise,R₂ amps/msec is shown in FIG. 3, which shows current and voltagesignals. R₂ is controlled by microprocessor 808 and can vary with wirefeed speed. R₂ is selected to be a rate that insures the current levelapproaches the value necessary to initiate the necking of the molteninterface between the wire and the weld puddle within the time requiredfor transfer of the molten ball through surface tension effects.

The necking of the interface refers to the molten column achieving across-sectional area smaller than the nominal cross-sectional area ofthe solid wire. This necking is a function of both surface tensionforces and the Lorentz force through which further reduction of the areais produced by the magnetic field which accompanies the current flowthrough this interface region.

The current magnitude increases (at the R₂ rate) until the current iscommanded to a level L. Upon reaching this level, controller 807commands a current rise at a rate of R₃, which is less than R₂. Thisrate of current rise is maintained until controller 807 determines theshort is about to clear. The event of clearing the short, i.e., thetransition from a short circuit to an arc, may be the most violentportion of the process and can produce the majority of spatter.

The explosive nature of this event is reduced, in the preferredembodiment, by lowering the magnitude of the current prior to or at theshort clearing, thereby limiting the power density. Early detection ofthe necking action is beneficial because the current level can then bereduced prior to the short clearing, thereby reducing spatter. Also, theconsistency with which the short clearing can be anticipated isimportant to effectively reduce spatter.

The present invention uses more information than can be obtained fromthe voltage waveform alone to quickly and consistently detect thenecking action. The interface between the wire and the puddle is used todetect the imminent short clearing. This is a region of high powerdensity due to the high current levels and the relatively small crosssectional area. The resistance of the interface region begins to rise asthe necking occurs and the cross sectional area decreases. This increasein resistance will cause a corresponding increase in the power densityin this region. Power=I²*R and R=(resistivity * dl/Pi * r²) where dl isthe length of the necking region and r is the radius of the neckingregion. Thus, as the radius of the necking region approaches zero, thepower density approaches infinity.

Controller 807 uses, in one embodiment of the invention, 1st derivativeof the power, dP/dt, to detect the short clearing event, in real time.However, the current rise during the time T_(RISE1) may cause the powerderivative hardware to attain the maximum output voltage level and staythere for a period of time. The slow recovery of the hardware circuitmakes the detection of a given threshold voltage indicating theprogression of the necking event difficult.

This problem is solved in another embodiment by subtracting a properlyscaled quantity related to the time rate of change of current. Thesignal used to detect the necking action is a control circuit voltageV_(c) as implemented by the hardware of the preferred embodiment. V_(c)is a calculated value and could be derived using a digital circuit. Thissignal is V_(c)=(k₁dP/dt−k₂*di/dt) (where k₁ and k₂ are scalars).Controller 807 determines that the necking has begun when V_(c) risesabove a level, V_(threshold). V_(threshold) is a threshold that may varywith weld conditions such as, for example, wire feed speed, wire type orwire size.

Using the scaled subtraction reduces the swing in V_(c) so that V_(c)does not saturate the hardware. Microprocessor 808 is enabled to acceptthe comparator signal indicating that V_(Threshold) had been reached inone embodiment. However, V_(c) is still greater than V_(threshold)during the T_(wet) time and shortly thereafter. Thus, controller 807does not sense the comparator output until after a delay, Dly₁, from thebeginning of T_(Rise). The delay is adjustable depending upon thewelding condition. An alternative embodiment uses a different scalingand different hardware (without the scaled subtraction).

The embodiments described above work much better than the prior art, butin accordance with the preferred embodiment an even earlier detection ofthe short clearing is provided. Controller 807, in the preferredembodiment, determines the derivative of the entire quantity definedabove. Thus, in the preferred embodiment V_(c)=d/dt(dP/dt−a*di/dt), andis plotted in FIG. 3. Again, when V_(c) crosses V_(threshold) controller807 determines, in real time, that the short is about to clear.Alternatives includes using other functions of dP/dt, using functions ofdV_(c)/dt instead of or with dP/dt, as well as using dR/dt, or higherorder derivatives of these parameters, or other functions of theseparameters, and combinations thereof.

Voltage and current feedback signals are used to obtain a power feedbacksignal. The voltage feedback is obtained from the gun head to the groundclamp on the work piece in the preferred embodiment. Current feedback ispreferably sensed using a current transducer, such as a LEM, in serieswith the current output, but located in the power source. Other feedbacklocations may be used.

Referring now to FIGS. 2A and 2B, a portion of analog circuit 809 whichis used to generate the signal “V_(c)” used by microprocessor 808 todetermine when the short is about to clear is shown. The arc voltage isprovided to lines 301 and 302. The arc voltage is scaled andpre-filtered by op amp A6-1, and the associated circuitry, resistors R94and R95 (200K ohms), R96 and R97 (0 ohms), R98 (10K ohms), R67 and R68(10K ohms) and capacitors C57 and C58 (0.001 μF). The voltage signal isfurther filtered and scaled by an op-amp A6-2 and resistors R45 (11Kohms), R46 (33.2K ohms), R47 and R48 (10K ohms), and capacitors C28 andC40 (0.001 μF). This provides a low noise signal of 1 volt/10 arc volts.The magnitude of the voltage signal is adjusted by an op-amp A6-3 andgain resistors R64 (10K ohms) and R63 (0-500K ohms). This signal isprovided on line 307 to a multiplier stage (described below withreference to FIG. 2B).

Similarly, a current feedback signal is received on line 303, where 1volt corresponds to 100 amps. The current signal is scaled andpre-filtered by op amp A3-1 and its associated circuitry, resistors R16(10K ohms), R17 (10K ohms), R40 and R41 (20K ohms), inductors L1 (1000μH) and L2 (188 μH), and capacitors C24 and C23 (0.001 μF). The currentsignal is then further filtered and scaled by an op-amp A3-2 and itsassociated circuitry, resistors R26 (10K ohms), R27 (33.2K ohms) and R30(10K ohms), and capacitors C14 and C16 (0.001 μF). This provides a lownoise signal of 1 volt/100 current amps. This signal is provided on line308, to a multiplier, after scaling by resistors R25 (10K ohms) and R32(0-50K ohms), which will now be described with reference to FIG. 2B.

A multiplier U1, shown on FIG. 2B, receives the voltage and currentsignals on lines 308 and 307, and provides an output representative ofthe power in the wire during the short (or at all times during which thefeedback is active). The power signal is provided through a resistor R42(1K ohms) to an op-amp A2 configured by a pair of capacitors C36 (0.068μF) and C27 (0.0022 μF), and a resistor R22 (51.1K ohms) to take thederivative of the power signal (dP/dt). Thus, the output of op amp A2,provided on a line 313, is a signal representative of the derivative ofthe power (dP/dt) in the wire during the short (or at other times thecircuit is active).

The derivative of the signal representative of the current (on line 310)is also taken. Specifically, an op amp A5-1 and associated circuitry apair of capacitors C33 (0.068 μF), and C37 (0.0022 μF) of FIG. 2B, aresistor R81 (30.1K ohms) and a zener diode D11 (4.7 V) are configuredsuch that the output of op amp A5-1 is a signal representative of thefirst derivative of current (di/dt) in the wire during a short (or atother times the circuit is active). The current derivative signal isscaled using an op amp A5-2 and a plurality of scaling resistors R82(10K ohms), and R62 (0-50K ohms).

The signals representative of the derivative of power and derivative ofcurrent are provided to op amp A5-3 through a resistor R35 (10K ohms)and resistor R43 (10K ohms), respectively. Op amp A5-3 is configured bya pair of resistors R84 (10K ohms) and R83 (10K ohms) to provide anoutput representative of the difference between the derivative of thepower and the derivative of the current (dP/dt−a*di/dt) during a short(or at any time the feedback is active).

Finally, this difference is provided to an op amp A5-4 which isconfigured by a pair of capacitors C29 (330 p F) and C56 (0.022 μF),resistors R44 (100K ohms) and R85 (1K ohms) and a zener diode D9 (4.7 V)to take a derivative. A diode and a 100K ohm resistor on the output ofA5-4 prevent the output from going negative. Thus, the signal on a line312 is representative of the derivative of the difference between thederivative of the power and the derivative of the current during a short(V_(c)=d/dt{dP/dt−a*di/dt}). This is the signal compared to thethreshold V_(Threshold). V_(Threshold) is a value set by microprocessor808, and, in the preferred embodiment, varies depending on wire feedspeed, wire size or type, or other parameters.

A plot of the signal on line 312 is shown in FIG. 3, along with the arccurrent and voltage. V_(threshold) is shown as a dashed line. The shortbegins when the voltage abruptly drops. The controller determines thatthe short is about to clear when d/dt{dP/dt−a*di/dt} crosses the dashedline (V_(threshold)). Thus, a technique for detecting when the short isabout to clear, by identifying a parameter that occurs at a predictabletime prior to the short clearing, is disclosed.

Most welders have an output stabilizer. The relatively large inductanceof a typical output stabilizer will “slow” the decay of the currentoutput, such that even if the current is commanded to low level when theshort is about to clear, the actual output current does not sufficientlyreduce prior to the short clearing. Thus, one aspect of the inventionincludes an “active” output stabilizer to help bring down the outputcurrent quickly after the detection of the short clearing.

Generally, as shown in FIG. 9, the active stabilizer includes an outputstabilizer 901 and a pair of coils 902 wound on a common core with mainstabilizer 901. A pair of switches 906 and 907 are in series with eachof coils 902, along with a pair of diodes. A dc source 908, and a pairof capacitors 909 and 910 are connected across coils 902 and switches906 and 907. Switches 906 and 907 are controlled by a controller 905such that current flows in coils 902 to create a flux opposing the fluxcreated by the output current. This reverses, in part or completely, thefield in stabilizer 901, and the output current quickly decreases. Theactive stabilizer is fired by controller 905 after the dP/dt circuitdetermines that the short is about to clear or is clearing. Thus, theoutput current quickly drops when the short clearing is detected.

While the present invention provides for much better short-clearingdetection than the prior art, it is still possible that either a shortclearing not detected, or the detection is a false positive.Accordingly, a safety net is provided in the event that the dP/dtdetection of the short about to clear doesn't work properly.

When the dP/dt circuit detects a short the active stabilizer is fired,the current is reduced, the dP/dt detection circuit is reset, and theoutput voltage is monitored to determine if the short actually clears.If, after a predetermined length of time, the short does not clear(indicated by the arc voltage failing to cross a threshold) then acurrent command is provided that causes the current to rise at a fixedrate to a value which is intended to clear the short. Subsequent clearramps may have faster rising current commands. Also, because the dP/dtcircuit was reset, the dP/dt is still compared to a threshold to detectwhen the short is about to clear. However, the threshold is increasedfor the subsequent comparisons to compensate for the increased currentfrom the ramp-up in the power level. If the new threshold is crossed,the commands above are repeated. Thus, a safety net for false positivesis provided.

Protection against a failure to detect a short clearing is alsoprovided. The arc voltage is monitored, and if the arc voltage indicatesthat a short has cleared then controller 807 advances the currentcommand to the next portion of the waveform.

As described above, another aspect of this invention is the ability tocontrol the arc length. This helps the short circuit transfer process beconsistently stable. The preferred embodiment uses arc voltage tocontrol arc length because there is a direct correlation between arclength and arc voltage. Generally, the arc voltage is compared to asetpoint. The arc length is determined to be more or less than a desiredlength based on whether the arc voltage is more or less than thesetpoint.

Also, a feedback relating to heat input to the wire, which correspondsto burn-off rate, is derived from a current feedback signal. Theinteraction of the two feedback loops—voltage for arc length and currentfor burn-off rate provides this control scheme with a stable arc.

The specific output circuitry that performs the arc length control isshown in FIG. 4. Instantaneous voltage feedback (V_(fbk)) and a voltagesetpoint (V_(set)) are provided to a differential amplifier A401.V_(set), is supplied by microprocessor 808 used to implement the controlscheme. A plurality of resistors R402-R405 (100K ohms) provide thedesired gain.

The output of op-amp A401 is provided to an op-amp A410 through a switchU301. Switch U301 removes the output of op amp A401 from the input of opamp A410 when the OCV exceeds a predetermined threshold set by an op ampA310, and a pair of resistors R311 (100K ohms) and R312 (68.1K ohms).Op-amp A410 is configured, along with a pair of resistors R411 (1Kohms), R412 (0-100K ohms), a switch U303, and a capacitor C302 (6 μF) tobe an integrator. Switch U303 is controlled by a capacitor C305 (1 μF),a resistor R306 (47.5K ohms) and a diode D307 to quickly clearintegrator A410.

A switch 413 is closed in an arc condition and is opened in a shortcondition. Switch 413, along with a switch 439, is controlled by an opamp A440. Op amp A440, along with a resistor R440 (619K ohms), and aresistor R443 (10K ohms) which determine when an arc or short ispresent. A diode D353 is used to reset the circuit in standby. Withoutswitch 413, the voltage error from integrator A410 during an arc outage,which quickly reaches the maximum, would adversely effect the stabilityof the process. Thus, when an arc outage occurs, the output of op-ampA410 is effectively removed from the control circuitry.

The output of integrator A410 is provided (when switch 413 is closed) toa summing junction of an op-amp A419. A plurality of resistors R420-R423(10K ohms) provide the appropriate scaling. The summing junction ofop-amp A419 also receives a signal indicative of a base command, whichhas a general form of the current shown in FIG. 3. These two inputs areused to set the long-term component of the current command. The basecommand is provided by microprocessor 808 through an op amp A425.

The remainder of the control circuitry will be described, below, afteran explanation of the short-by-short, current feedback based, control.

A control signal derived from current feedback, which corresponds toheat input to the wire and burn-off rate, provides the short-by-shortcontrol. The short-by-short control entails monitoring of the heat inputinto the wire. Given certain information regarding the type of wirebeing consumed into the weldment, the rate at which heat must be inputinto the wire in order to maintain the burn-off rate is calculated.

Some prior art patents “totalize” the heat input into the weldment byintegrating total power input with respect to time and comparing thistotal to a preset value. This requires input of both the voltage andcurrent feedback signals for the power calculation. However, analysis ofthe physics involved with the melting of the wire shows that theburn-off of the wire is independent of the arc voltage. The heatinput/sec into the wire is given by the following equation:

Q _(Wire) /sec=[(V _(anode) +W.F.+3 kT/2 e)*I(t)+I ²(t)l*ρ/A], where:

V_(anode)=anode voltage drop

W.F.=work function of the metal

3 kT/2 e=thermal energy of the electrons impinging on the wire

l=contact tip to arc

ρ=resistivity of the metal

A=cross sectional area of the wire

I(t)=instantaneous current

It can be seen from these equations that the melting rate of the wirecan be expressed in terms of I(t) only, independent of V(t). The firstterm of this equation applies only in the arc mode of the process, whilethe second term is applicable in both the arc and short circuit modes.

The amount of energy required to melt a given size and type of wire witha fixed feed rate can be determined with the following equation:

Q _(req) =R _(dep)*(H _(m)+(T _(drop) −T _(amb))*C _(p))*t _(tot),where:

R_(dep)=wire mass deposition rate

H_(m)=latent heat of melting for the wire

T_(drop)=temperature of the molten drop

T_(amb)=ambient temperature of the wire

C_(p)=heat capacity of the wire

t_(tot)=average period of a short/arc sequence

The preferred embodiment uses the arc physics to insure that the energyinput required to maintain the melting of the incoming wire is suppliedin a consistent manner. This means that variations of feed rate due tothe operators' movements will be accounted for and the instantaneousburn-off rate will be adjusted. The circuit employed in achieving thiscontrol (implementing these equations) is shown in FIG. 4, and is partof analog circuit 809.

The current feedback signal (I_(fbk)) is provided on line 309 from theBessel filter A3-2 of FIG. 2. This signal is connected to both inputs ofa multiplier U424 to yield an output proportional to I²(t). The I²(t)signal is then scaled by a pair of gain setting resistors R431 (1K ohms)and R432 (0-100K ohms) of an amplifier A430 to produce a representationof the resistive heating in the wire. The gain of amp A430 is equated tothe resistance of the wire stick out. The output of amp A430 is providedto a summing node, where it will be added to two other components.

The current feedback input, I_(fbk), is also provided to an amplifierA435. Amplifier A435 has a gain set by a pair of resistors R436 (5Kohms) and R437 (0-10K ohms), and represents V_(anode)+W.F.+3 kT/2 e.This is the coefficient of the arc contribution to the wire heat inputin the Q_(Wire) equation above.

The output of amp A435 is switched into the summing node by an analogswitch U439. Switch U439 insures that this portion of the heat input isprovided only during an arc, and not during a short circuit. The voltagefeedback signal (V_(fbk)) is provided to a comparator A440. ResistorsR440 (619K ohms) and R443 (10K ohms), and using a signal frommicroprocessor 808 are used for the arc/no arc determination. Thus, theoutput of amp A435 is provided to the summing node only during an arc.

The third input to the summing node is an average required heat input orburn rate, J_(set), which comes from microprocessor 808. J_(set) is apredetermined value of required power input into the wire to sustainburn-off at a given feed rate. Its value is feed-rate-dependent and isadjusted by microprocessor 808 as wire feed speed is adjusted. J_(set)is provided to the non-inverting input of an op amp A350, which hasscaling resistors R351 (10K ohms) and R352 (20K ohms).

The instantaneous heat input rate, determined from the outputs of ampsA430 and A435 are compared to with the required average input rate,J_(set), by an amplifier A446 and a plurality of resistors R447, R448,R450, and R454 (10K ohms).

The output of amp A446 is provided through an analog switch 451 to avoidpotential start up transient problems. Switch 451 is controlled bymicroprocessor 808 to be closed for welding and open for standby. Theinstantaneous heat rate differences from amp A446 are integrated by anop amp A452, resistors R453 (0-100K ohms), R343 (4.75K ohms), and R342(100K ohms), and a capacitor C454 (6 μF). The integrated value is scaledby an op amp A460, and resistors R462 (0-100K ohms), R340 (10K ohms),and R341 (10K ohms) to produce a command correction signal that adjuststhe current command in an attempt to maintain constant burn-off (or massdeposition rate) of the wire. This represents a portion of theinstantaneous or short-by-short control described generally above.

This correction command is provided to summing op amp A419 throughresistor R422 and a pair of op amps A320 and A324. Op amp A320, andresistors R321 and R322 (10K ohms), invert the signal. Op amp A324 hasan adjustable gain controlled by a pair of switches U327 and U328, andresistors R323 (10K ohms), R325 (10K ohms), and R329 (0-100K ohms).Switch U327 is controlled by an op amp A333, which opens switch U327during an arc. Switch U328 is closed by U303 during standby. Thus, aplurality of gains are provided.

The base command is provided to op amp A419 through resistor R421, andthe integrated voltage error signal is provided through resistor R420.The output of op amp A419 is provided to an op amp A467. Op amp A467,along with resistors R468 (10K ohms), R470 (10K ohms), and capacitorC310 (270 p F) return the modified command to its' proper polarity. Adiode D311, a resistor R465 (10K ohms) and an op amp A309 set theminimum current command output.

Another aspect of this invention is to provide an overall stability tothe short arc process by controlling the pre-heating of the wire. Theinventors have determined that the MIG process is intrinsicallyoscillatory. The oscillations are of a low frequency, typically in therange from 2 to 10 Hz. They result from the non-uniform pre-heating ofthe wire, which is often caused by either spot heating, or by theoperator changing the arc length or stick-out length, as willbe-described below.

The current is transferred to a welding wire 501 through a dynamicinterface within a contact tube 502 as shown in FIG. 5 in most MIGwelding systems. A contact area 504 is a high current density region dueto the relatively small area of current flow. This high current densityregion has the potential to cause spot heating in the wire.

Also, problems can arise if there is a perturbation in the process whichcauses a period of higher current to flow into the wire. For example,operator movement can momentarily increase the wire delivery rate andthereby requires additional current to melt the wire. When additionalcurrent is provided a hot spot in the wire is produced. This spotheating causes the resistance in that particular portion of the wire toincrease. This increased resistance further enhances the I²*R heat inputinto the wire in that region as current continues to flow in subsequentshort circuit and arc sequences. The end result is an area of localizedheating in the wire. When this area reaches the weld puddle, the amountof wire which is melted following a short circuit will be greater thanusual.

The wire can melt back excessively in extreme cases, and result in aflare-up of the arc which is detectable to the operator. This flaringdetracts from the overall stability of the process and is undesirable.Furthermore, the long arc time caused by spot heating, results in a longtransit time of the wire back to the puddle. During this transit time,the current is low, (A_(bk)) and therefore, the I²*R heating at thecontact area in the contact tube is low. This produces a relative coldspot in the wire which begins to travel toward the puddle. As this coldregion of wire approaches the weld puddle, the size of the molten ballformed after the short clears, decreases. Also, the time spent in thearc mode decreases. This shift in time from the arc to short circuitincreases the overall I²*R heating of the wire. This increased I²*Rheating produces a localized hot spot in the wire near the contact tube,bringing the cycle back to the beginning. Thus, this process may becyclic in nature.

The frequency of this cyclic phenomenon is related to a number offactors. Chief among these are the stick out length (503 of FIG. 5) andwire feed speed. The fundamental frequency of oscillation is representedby the inverse of the transit time of a section of wire equal to thelength of the stick out, traveling at a velocity equal to the wire feedspeed. Data demonstrating this relationship for a 1″ stick out is shownin FIG. 6. It should be noted that higher modes of this fundamentalfrequency could conceivably be excited.

Prior art short circuit control algorithms generally cannot adjust forsuch “pre-heating” until it changes the arc voltage/arc length. However,by the time the problem manifests itself in this manner, it is too lateto change the result. Thus, advanced knowledge of the heat input intothe end of the wire is used to compensate for variations in pre-heatingportions of the wire.

The power input into the arc can be regulated properly to avoid theflaring and stubbing cycles if the state of the wire which will beexposed to the arc is known. The state of the end of the wire can bedetermined from its current carrying history. Information regarding thestick out length is used so that one can go back in time the properdistance to correctly ascertain how much current a segment of wire(i.e., a small linear portion of the wire) has carried.

The length of stick out is determined by summing the output from voltageerror integrator A410 through a resistor R476 (10K ohms) with the outputof J_(set) error op amp A460 through a resistor R477 (10K ohms) by anop-amp A480, including a resistor R478 (20K ohms), a resistor R481 (10Kohms), and a capacitor C482 (5.6 μF).

The output of op amp A480 generates a relatively linear function whenplotted with stick out as the independent variable. The inverse functionyields a linear relationship between the output of op amp A480 and theactual wire stick out in units of length. The slope and intercept ofthis line can be stored in microprocessor 808 for a given wire size,type, feed speed, etc. Thus, all the information needed to determine thewire stick out is available to microprocessor 808 for a given weldingcondition.

The heat input into the wire is determined by treating the wire stickout as a series of small segments (see FIG. 7). The I²*R heat input intoeach segment is found by taking multiple samples of the I²*R output atamp A430 and summing these over a duration of time less than theshort-arc period (cycle process). This sum then represents the heatinput into each segment along the stick out. An array stores the heatinput information so that a cumulative sum is maintained for eachsegment. The segment which contains the heat input information for theend of the wire is determined by going back in time an amount based uponthe stick out, as measured by the output of op amp A480. The magnitudeof the sum of the wire end segment is used, in this embodiment, todetermine the amplitude of the current level during the arc time. Thesum is compared to a predetermined heat level and the magnitude of thecurrent during the arc is increased or decreased in proportion to theerror present. Other aspects of the current waveform, such as Hldt, rateof rise, or R₁, could be utilized to control the arc heat input basedupon the heat at the end of wire in other embodiments.

Another aspect of this invention is providing a stop algorithm thatdoesn't allow the formation of a large ball at the end of the wire. Thisis accomplished using microprocessor 808. Specifically, a stop signal isreceived by microprocessor 808 (for example, when the user ends theprocess). Microprocessor 808 then commands the motor to come to a stop.Feedback from wire feeder 801, derived from a tachometer, allows themicroprocessor to determine the wire feed speed. Microprocessor, 808commands a low CV command until a predetermined wire feed speed isreached (about 200 IPM in the preferred embodiment). Alternatively,after receiving the stop command the process parameters are ramped downuntil the wire feed reaches 75 IPM. When the predetermined wire feedspeed is reached controller 807 sends special current commands to thepower source.

Controller 807 monitors the arc voltage, and when a short is detected(indicated by a drop in arc voltage) a rising current is commanded(similar to the response of the normal welding process). When the arcvoltage reaches the predetermined threshold (indicating that the shorthas cleared) the rising current command is terminated, and very lowcurrent (about 0-10 amps in the preferred embodiment) is commanded. Withvery low current, very little ball formation occurs. Thus, if the wiredoes not advance further, and does not touch the puddle, a large ball isnot formed on the end of the wire.

If, however, the wire continues to advance and touches the puddle, orthe puddle flows back and touches, the routine is repeated, and again, alarge ball is not left on the wire. This algorithm continues to repeatuntil the wire stops, and a large ball is not formed. It should be notedthat this algorithm does not consume much wire since large balls are notformed. Therefore, this process cannot be activated until very littlewire advancement is expected.

The voltages used to determine if the wire is shorted or not arereferenced to the current flowing through the wire in the preferredembodiment. Thus, if a medium voltage level is detected and the selectedcurrent magnitude is low, then the short has cleared. However, that samearc voltage at a high selected current level might indicate the shortstill exists, and that the wire is merely getting hot. Thus, the voltagethreshold is adjusted by microprocessor 808 based on the selectedcurrent level.

One alternative is to provide a stop signal to power source 805 frommicroprocessor 808 that overrides a minimum current setting during thestopping time (the minimum current is set for a number of low currentapplications where the arc is in danger of being extinguished). Thencontroller 807 allows the power source to continue to do its constantvoltage (CV) control, but commands a much lower voltage and the arc timecurrent would naturally be less. Other alternatives include controllingthe braking of the wire feed motor, along with the electrical output ofthe power source. This aspect of the invention is readily adapted toprocesses other than short circuit transfer welding, such as an arcspray process, and with other control schemes.

The algorithm for stopping is also disclosed in a U.S. PatentApplication entitled Method and Apparatus for Stopping a WeldingProcess, filed on even date herewith by Holverson and Mehn, and assignedto the owners of this application, which is hereby incorporated byreference.

Numerous modifications may be made to the present invention which stillfall within the intended scope hereof. Thus, it should be apparent thatthere has been provided in accordance with the present invention amethod and apparatus for short circuit transfer welding that fullysatisfies the objectives and advantages set forth above. Although theinvention has been described in conjunction with specific embodimentsthereof, it is evident that many alternatives, modifications andvariations will be apparent to those skilled in the art. Accordingly, itis intended to embrace all such alternatives, modifications andvariations that fall within the spirit and broad scope of the appendedclaims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An apparatus for weldingcomprising: a power source having a power control input and a poweroutput; a welding output; an output inductance circuit, disposedelectrically between the power output and the welding output, and havingan inductance control input, further having a first inductance and asecond inductance responsive to the control input; a controller, coupledto the power control input and the inductance control input; and afeedback circuit coupled to a short-detection input of the controller,wherein the controller closes the switch in response to theshort-detection input.
 2. The apparatus of claim 1, wherein the outputinductance circuit comprises: an output winding being disposedelectrically between the power output and the welding output; and atleast on switch disposed to control the inductance of the outputwinding, wherein the switch has a control input coupled to thecontroller.
 3. The apparatus of claim 1, wherein the output inductancecircuit comprises: an output winding being disposed electrically betweenthe power output and the welding output; at least on control winding,wherein the output winding and the control winding share a magneticcore; and a switch disposed to control current flow through the at leastone control winding, wherein the switch has a control input couple tothe controller.
 4. The apparatus of claim 1, wherein the at least onecontrol winding and the output winding are wound such that current flowin the control winding creates a first flux that opposes a second fluxcreated when current flows in the output winding.
 5. The apparatus ofclaim 1, wherein the output inductance circuit further comprises astabilizer power source coupled to the at least one control winding suchthat, in the event the switch is closed, current flows from thestabilizer power source to the at least one control winding, or from theat least one control winding to the power source.
 6. The apparatus ofclaim 1, wherein the at least one control winding includes two controlwindings.
 7. An apparatus for welding comprising: a power source, havinga power control input and a power output; a welding output; an activestabilizer, disposed electrically between the power output and thewelding output, and having an inductance control input, including anoutput winding being disposed electrically between the power output andthe welding output, at least one control winding, and a switch disposedto control current flow through the at least one control winding, and astabilizer power source coupled to the at least one control winding; anda controller, coupled to the power control input and the inductancecontrol input; a feedback circuit coupled to a short-detection input ofthe controller, wherein the controller closes the switch in response tothe short-detection input.
 8. The apparatus of claim 7, wherein theactive stabilizer comprises an output winding being disposedelectrically between the power output and the welding output and atleast one switch disposed to control the inductance of the outputwinding.
 9. An apparatus for welding comprising: means for providingpower to a power output; a welding output; means for providing acontrollable inductance between the power output and the welding output,including mean for inducing a first flux in a magnetic core when poweris provided to the welding output, means for inducing a second flux,opposing the first flux, in the magnetic core, and means for controllingcurrent flow in the means for inducing a second flux, connected to themeans for controlling; means for controlling the means for providingpower and the means for providing a controllable inductance, coupled tothe means for providing power and the means for providing a controllableinductance; and means for detecting an output short-circuit, connectedto the means for controlling, wherein the means for controlling furtherincludes means for controlling the means for providing a controllableinductance in response to the means for detecting.
 10. The apparatus ofclaim 9, wherein the means for providing a controllable inductancefurther comprises means for providing current to the means for inducinga second flux.
 11. An apparatus for welding comprising: means forproviding power to a power output; a welding output; means for activelystabilizing the welding output, dispose electrically between the poweroutput and the welding output; means for controlling the means foractively stabilizing, connected to the means for actively stabilizing;and means for detecting a short circuit of the welding output, coupledto the means for controlling, wherein the means for controlling furtherincludes means for controlling the means for actively stabilizing inresponse to the means for detecting.
 12. A method of welding comprising:providing welding power; controlling an output inductance, having atleast first and second inductances and disposed electrically between thepower output and the welding output; opening and closing a switchconnected to a control winding, wherein the control winding shares amagnetic core with an output winding disposed electrically between thepower output and the welding output; and detecting a short circuit ofthe welding output and controlling the inductance in response thereto.13. The method of claim 12, further comprising inducing a flux in themagnetic core that opposes a flux induced by the output winding.